In a long-distance optical transmission system, an optical repeater called a 3R optical repeater, which converts an optical signal into an electric signal for retiming, reshaping and regenerating the optical signal, has been used. At present, however, an optical fiber amplifier has been put into practical use to a large extent and R and D effort in this field is focused on the optical amplifying and repeating system using the optical amplifier as a linear repeater. By replacing the conventional 3R optical repeater with an optical amplifying repeater, a number of elements in the repeater can be reduced conspicuously, reliability of the system can be improved, and remarkable reduction of cost is realized.
Moreover, in order to realize a large capacity optical transmission system, attention is paid to a wavelength division multiplexing (WDM) optical transmission system in which two or more optical signals having different wavelengths are multiplexed onto a single optical fiber transmission line.
In a WDM optical amplifying and repeating transmission system in which the WDM optical transmission system and optical amplifying repeating transmission system are combined, the two or more optical signals having different wavelengths can be amplified at a time by optical amplifiers and thereby a large-capacity and long-distance transmission can be realized with a simplified and economical configuration.
In the wavelength division multiplexing repeating transmission system, management of the chromatic dispersion on the optical fiber transmission line is employed to reduce deterioration of transmission characteristics due to a nonlinear effect caused in the optical fiber transmission line (i.e., a single-mode optical fiber). FIGS. 17(a) and 17(b) show an example of chromatic dispersion in an experimental transmission system reported, for example, in an article of N.S. Bergano et al., xe2x80x9cWavelength division multiplexing in long-haul transmission systemxe2x80x9d, IEEE Journal of Lightwave Technology, vol. 14, no. 6, pp. 1299-1308, 1996. In this experiment, a dispersion-shifted fiber (DSF) having a zero-dispersion wavelength xcex0 of 1585 nm in the length of about 900 km and the single mode fiber (SMF) having a zero-dispersion wavelength xcex0 of 1310 nm in the length of about 100 km are used in combination. The average zero-dispersion wavelength of the DSF in the length of about 900 km and the SMF in the length of about 100 km is about 1558 nm. Wavelength xcex1 of an optical signal ranges from 1556 nm to 1560 nm. The chromatic dispersion of the DSF is about xe2x88x922 ps/nm, group velocities of the optical signals and spontaneous emission generated by optical amplifiers are different, and group velocities among optical signals are different. Thus, the period in which the nonlinear effect mutually affects these optical signals and the spontaneous emission can be shortened. Additionally, deterioration of the transmission characteristics due to the four-wave-mixing (FWM) and cross phase modulation (XPM) can be reduced.
In FIGS. 17(a) and 17(b), the DSF occupies about 90% of the optical fiber transmission line and is called a transmission fiber. An optical fiber for compensating for the chromatic dispersion of such a transmission fiber is called a dispersion compensation fiber.
FIG. 18 shows a proposed method to manage the chromatic dispersion of an optical fiber transmission line. In an example of the chromatic dispersion reported in an article of Y. Hayashi et al., xe2x80x9cVerification of four-wave mixing suppression in WDM transmission experiments on the FSA commercial system with dispersion-managed optical fiber cablexe2x80x9d, OFC ""96 TuI7, pp. 49-50, 1996, wherein chromatic dispersion administration is introduced to make zero the chromatic dispersion accumulated in each repeating span.
Therefore, self phase modulation (SPM) in addition to the chromatic dispersion is a major cause of limitation on the transmission characteristics.
Important requirements for an optical fiber transmission line used in the wavelength division multiplexing transmission system are listed below.
(1) Low transmission loss
(2) Large nonlinear effective cross-sectional area
(3) Nomatching between a wavelength of an optical signal and a zero-dispersion wavelength
(4) Average chromatic dispersion in a direction of transmission distance is negative
(5) Chromatic dispersion compensation interval is about 10 times the repeating interval
In order to realize further large-capacity and long-haul transmission, focused should be placed on reducing the nonlinear effect of the optical fiber transmission line in the wavelength division multiplexing optical amplifying and repeating transmission system.
In order to attain such an object, an optical fiber having a nonlinear effective cross-sectional area (large effective area fiber, LEAF) is used in the following articles.
1) Y. Liu et al., xe2x80x9cSingle-mode dispersion-shifted fibers with large effective area for amplified systemxe2x80x9d IOOC ""95, PD2-9, 1995.
2) P. Nouchi et al., xe2x80x9cLow-Loss single mode fiber with high nonlinear effective areaxe2x80x9d, ThH2, OFC ""95.
3) J. P. Hamaide et al., xe2x80x9cExperimental 10 Gbit/s sliding filter guided soliton transmission up to 19 Mm with 63 km amplifier spacing using large effective area fiber managementxe2x80x9d, Th.A. 3.7., ECOC ""95.
Moreover, M. Suzuki et al. have conducted an experiment of wavelength division multiplexing optical amplifying and repeating transmission using an optical fiber having a large nonlinear effective cross-sectional area (large core fiber, LCF). This report is entitled xe2x80x9c170 Gb/s Transmission over 10,850 km using large core transmission fiberxe2x80x9d, OFC"" 98 PD17, 1998. Configuration of the optical transmission fiber used in this transmission experiment is shown FIG. 19 and its characteristics are shown in Table 1.
In the first half of the repeating span, LCF is used and in the second half of the repeating span, conventional dispersion-shifted WDM fiber (WDMF) is used. LCF has an advantage in that the nonlinear effective cross-sectional area is large and disadvantages in that the transmission loss is large and the dispersion slope is also large. Meanwhile, WDMF has an advantage that transmission loss is small and dispersion slope is also small and a disadvantage in that the nonlinear effective cross-sectional area is small. This disadvantage can be compensated for by using LCF and WDM fiber in combination.
However, the essential disadvantage is that the transmission loss of LCF is large. Particularly, its transmission loss becomes large rapidly if an external force is applied to the optical fiber or such an optical fiber is bent. Thus, the optical fiber cable provided for installation has a higher probability of an increase in the transmission loss. Tsuchiya et al. have discussed the waveguide structure parameter of a uniform core optical fiber having the zero-dispersion wavelength of 1.5 xcexcm. The report is entitled xe2x80x9cDispersion-free single-mode fiber in 1.5 xcexcm wavelength regionxe2x80x9d, Electronics Letters, vol. 15, No. 15, pp. 476-478, 1979. The relationship between relative core-cladding index difference and core radius to realize zero-dispersion is shown in FIG. 20. The sections a-b of the solid lines in the Figure indicate the optimum values and the dotted lines indicate the higher order mode cutoff condition. Here, there is a tendency that the longer the zero-dispersion wavelength is, the smaller the core system becomes and the larger the specific refractive index becomes. This results in a higher probability of an increase of the bending loss.
Therefore, it is a big issue in the wavelength division multiplexing optical amplifying and repeating transmission system that a low transmission loss and a nonlinear effective cross-sectional area must be realized simultaneously.
An object of the present invention is to provide an optical fiber transmission line for wavelength division multiplexing transmission system in which the above-mentioned disadvantages are eliminated.
The above object of the present invention is achieved by an optical transmission system for transmitting an optical signal through an optical fiber transmission line, the optical fiber transmission line comprising: a first optical fiber transmitting the optical signal and having a first zero-dispersion wavelength shorter than a wavelength of the optical signal; and a second optical fiber transmitting the optical signal from the first optical fiber and having a second zero-dispersion wavelength longer than the wavelength of the optical signal. A first nonlinear effective cross-sectional area of the first optical fiber is larger than a second nonlinear effective cross-sectional area of the second optical fiber.
The optical transmission system may be configured so that chromatic dispersion values and lengths of the first optical fiber and the second optical fiber are so selected that the total chromatic dispersion value of the first and second optical fibers becomes a negative value.
The optical transmission system may be configured so that the first nonlinear effective cross- sectional area of said first optical fiber is 70 xcexcm2 or more and the second nonlinear effective cross-sectional area of said second optical fiber is smaller than 70 xcexc2.
The optical transmission system may be configured so that the chromatic dispersion value of said first optical fiber is selected from +1 to +2 ps/nm/km, and the chromatic dispersion value of said second optical fiber is selected from xe2x88x923 to xe2x88x922 ps/nm/km.
The above object of the present invention is also achieved by an n optical transmission system for transmitting an optical signal through an optical fiber transmission line, the optical fiber transmission line comprising: a first optical fiber, transmitting the optical signal, and having a first zero-dispersion wavelength shorter than a wavelength of the optical signal and a first nonlinear effective cross-sectional area; a second optical fiber, transmitting the optical signal from the first optical fiber, and having a second dispersion wavelength longer than the wavelength of the optical signal and a second nonlinear effective cross-sectional area smaller than the first nonlinear effective cross-sectional area; and a third optical fiber, transmitting the optical signal from the second optical fiber, of which a third chromatic dispersion value and length is selected to cancel a total chromatic dispersion value of the first and second optical fibers.
The just above optical transmission system may be configured so that the chromatic dispersion values and lengths of the first optical fiber and the second optical fiber are so selected that the total chromatic dispersion value of the first and second optical fibers becomes a negative value.